The present invention relates to organic electroluminescence display devices, and more particularly to an increase in brightness in an organic electroluminescence display device with organic electroluminescence elements of plural colors stacked on an insulating substrate that makes full color display.
As display devices of flat panel type, liquid crystal displays (LCD), plasma display panels (PDP), field emission displays (FED), and organic electroluminescence displays (OLED) are commercially practical or in the stage of a study for practical use. Particularly, the organic electroluminescence display devices are greatly promising as a representative of display devices of spontaneous emission type that are thin and light in weight. The organic electroluminescence display devices are classified as a so-called bottom emission type and a top emission type. The present invention is described with respect to organic electroluminescence display devices of an active matrix type. However, a light emitting layer structure holds true for organic electroluminescence display devices of a simple matrix as well.
FIGS. 7A and 7B are drawings for explaining the structure of an organic electroluminescence display device of bottom emission type. FIG. 7A is a sectional view for explaining an overall structure, and FIG. 7B is a sectional view for explaining the structure of a unit pixel. In the organic electroluminescence display device of bottom emission type, a thin film transistor TFT is provided on the principal surface of an insulating substrate SUB being preferably a glass substrate, a first electrode or one electrode (hereinafter referred to as a bottom electrode or a transparent electrode (ITO, etc.) as a pixel electrode) BEL is formed via a contact hole formed on an insulating film INS. The bottom electrode BEL is formed independently for each of unit pixels.
A bank BNK formed from an insulating material is provided over the region in which the thin film transistor TFT is formed, separates one adjacent unit pixel from another, and serves to accommodate an organic light emitting layer ILL that emits light on application of an electric field. A reflective metallic electrode as a second electrode (common electrode) or another electrode, that is, an upper electrode UEL is stacked over the organic light emitting layer ILL. The insulating substrate SUB having organic electroluminescence elements thus constructed on its principal surface is isolated by a sealing can CAV from the external atmosphere, and is sealed with a sealant such as an adhesive. A desiccant DSC is stored in the inside sealed by the sealing can CAV.
By applying an electric field to between the bottom electrode BEL as an anode and the upper electrode UEL as a cathode, carriers (electrons and positive holes) are injected to the organic electroluminescence elements composed of organic multilayer films, and the organic multilayer films emit light. Light emitted from the organic electroluminescence device is emitted as display light via the insulating substrate SUB. A full-color image is displayed by arraying a plural number of color pixels in a matrix form, which are the red (R), green (G), and blue (B) unit pixels of the organic electroluminescence elements.
FIGS. 8A and 8B are drawings for explaining the structure of an organic electroluminescence display device of top emission type. FIG. 8A is a sectional view for explaining an overall structure, and FIG. 8B is a sectional view for explaining the structure of a unit pixel. In the organic electroluminescence display device of top emission type, the bottom electrode BEL corresponding to one electrode of the above-described organic electroluminescence display device of bottom emission type is constructed with a reflective metallic electrode, and the upper electrode UEL being another electrode is constructed with a transparent electrode such as ITO, organic multilayer films emit light when an electric field is applied to between the both, and the emitted light L is emitted from the upper electrode UEL. In the top emission type, a transparent plate being preferably a glass plate is used as the sealing can in the bottom emission type, and desiccant DSC is a transparent material or disposed in a portion that does not cut off display light. Other parts of the structure are almost the same as those in FIGS. 7A and 7B.
Technology is known for displaying full-color images in combination of color filters of red (R), green (G), and blue (B) with a white organic electroluminescence element, instead of organic electroluminescence elements of red (R), green (G), and blue (B).
In Japanese Patent Laid Open No. 2004-311440, an R-G-B-W method is disclosed by which an organic electroluminescence element of white (W) is added to organic electroluminescence elements of red (R), green (G), and blue (B) to increase display brightness.
In the R-G-B-W method disclosed in the Japanese Patent Laid Open No. 2004-311440, separate organic electroluminescence films to emit light of red (R), green (G), blue (B), white (W) are formed with the same layer thickness, and four organic electroluminescence film formation processes such as evaporation are required. Since a conventional organic electroluminescence film to emit white (W) light is generally formed from a mixture of luminescent materials of two colors, an emission spectrum tends to be gentle and color balance adjustment (color purity adjustment) is difficult. In the case of an organic electroluminescence display device with color filters of red (R), green (G), and blue (B) combined with a white organic electroluminescence element, since part of light emitted from the organic electroluminescence elements is absorbed by the color filters, light extracting efficiency decreases.